I started to vote on your poll but I think that the three things you identified: economy, injuries, and families that just don't want to go (which I am thinking is closely related to the economy, though there may be families who feel that the particular nationals doesn't warrant the expense when viewed in light of the college exposure factor) are pretty much equal. So I'd like to be able to vote for all three, which is not possible on the poll.
BTW, if the team has always indicated that it will go to a particular nationals if it gets a berth, I would hope that any decision by a family not to attend would be based on changed economic circumstances and not on other factors, barring exigent circumstances. If a majority of the players planned on and want to attend a nationals, I think it is a shame to put that team in the position of not being able to go because at the relatively last minute some players changed their minds. JMHO and I know that if the team didn't qualify for its preferred nationals there are some rational arguments against my opinion.